This invention is concerned with gravity stable surfactant flooding, particularly in vertical reservoirs which have been depleted by gas drive or gas expansion recovery methods.
Many different enhanced oil recovery techniques have been suggested over the years. Quite a few of these methods have been tried in the field with varying results. One of the more successful techniques, particularly in vertical reef reservoirs, has been the use of a gas drive to push hydrocarbons through a formation. Such methods leave an oil saturation in the swept portion of the reservoir at or near residual oil saturation to gas. The majority of mobile fluid in place is gas.
Some substantially vertical reservoirs, such as the vertical reef reservoirs in Western Canada have been subjected to a miscible flooding technique referred to as miscible blanket flooding. This is a vertically downward moving flood which is especially suitable for use in thick reservoirs having a vertical thickness in excess of 50 feet or more.
In miscible blanket flooding, a solvent is injected into the upper portion of the reservoir. After the injection of a predetermined volume of solvent which is sufficient to form a thin layer or blanket on top of the oil saturated portion of the formation, a drive fluid is injected into the upper portion of the formation to displace the slug or blanket of solvent vertically downward. Ideally, vertical miscible blanket flooding contemplates the establishment of a discreet, relatively thin layer of solvent which is spread completely across the top of the formation. U.S. Pat. Nos. 3,850,243 and 4,249,607, and Canadian Pat. Nos. 1,227,419 and 1,227,420 describe such processes.
U.S. Pat. No. 3,455,389 discloses a surfactant flooding scheme in a dipping reservoir with a water drive. In this secondary recovery method, a surfactant slug is injected at the interface between the aquifer and the formation hydrocarbons. U.S. Pat. No. 4,372,381 discloses the injection of a hydrocarbon solvent slug in a dipping reservoir between an aquifer and formation fluids.
Another injection and production process for a dipping reservoir having water and gas zones is disclosed in U.S. Pat. No. 3,519,076. This reference teaches the injection of water into the water zone to move the water zone upwards, driving hydrocarbons to a production well. After the water zone has moved to shut off oil production from the production well, the production well is converted to gas injection and oil is produced from another production well.